The crescent Moon anchors a bright lineup this evening. The star Spica is to the right of the Moon, with the planet Saturn about the same distance to the right of Spica. They’re low in the western sky as night falls, and set by around 9:30 or 10.

The bright pinpoint of light that we see as Spica actually consists of two stars that are locked in a tight orbit around each other. Both stars are much bigger, hotter, and heavier than the Sun.

The stars are separated by about 10 million miles — a mere hop by astronomical standards.

In fact, the stars are so close together that they have a dramatic effect on each other. Their gravitational pull is so strong that each star causes the other to bulge outward. It’s like the tides in Earth’s oceans — a bulge of water caused by the gravitational pull of the Moon. The bulges on the stars are bigger, though, so seen in profile, the system would look at bit like two eggs with the narrow ends pointing toward each other.

The two stars turn on their axes at different speeds, though, so hot gas in their upper layers move through the bulges, then around to the back of the stars. That stirs up the outer layers, creating effects that make it hard for astronomers to see what’s really going on. So they create models that try to explain what’s happening, and compare those models to what they actually see. It’s a process that’s ongoing — the never-ending process of learning about the universe.

Script by Damond Benningfield, Copyright 2011

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